1. Field of the Invention
The invention relates to a process and apparatus for removing pollutants, such as carbonaceous materials, from gas streams, such as combustion exhaust streams, in which the stream is exposed to a laser beam having a frequency that is absorbed selectively by the pollutant.
2. Description of the Prior Art
U.S. Pat. Nos. 3,977,952 and 4,097,349 describe processes for removing contaminants or pollutants from combustion gases that involve exposing the gas to electromagnetic radiation. In the process of U.S. Pat. No. 3,977,952, oxygen and water containing a small amount of HCl are added to the gas and the mixture is exposed to light having a wave length of 20 to 600 nm. Photo-induced reactions occur that convert the carbon, nitrogen, and sulfur to gases that are allowed to escape from the aqueous phase.
U.S. Pat. No. 4,097,349 teaches a process for removing sulfur dioxide and nitrogen-oxygen compounds from combustion gases by a photochemical reaction of such materials with a reactive hydrocarbon. Specifically, oxygen and an olefinic hydrocarbon are added to the combustion gas and the mixture is irradiated with electromagnetic radiation having a wavelength of 1500 A.degree. to 7500 A.degree.. The radiation excites the sulfur dioxide and causes free radical formation, thereby promoting conversion of the reactants into a particulate material that is removed by conventional separation techniques.
High energy laser beams have been used to fix nitrogen. U.S. Pat. No. 4,167,463 teaches a nitrogen fixation process in which air and nitrogen are charged under pressure into a chamber having a window. A high energy laser beam is passed through the window into the chamber thereby heating the mixture to create a plasma in which nitric oxide is formed. The plasma is quenched rapidly to prevent decomposition of the nitric oxide to nitrogen and oxygen and the nitric oxide is removed from the quenched gas by scrubbing with water.
U.S. Pat. No. 4,226,369 describes laser treatment of coal combustion gas to destroy particles above 2-5 microns to make the gas suitable for driving turbines. The treatment apparatus includes a chamber with intake and exhaust outlets for the gas. A plurality of lasers flank opposite sides of the chamber. The beams from the laser are directed into the chamber through windows in the chamber sides at a slight (1.degree. to 2.degree.) angle to the gas flow to form a Chevron pattern of illumination.
Lasers have also been used in photolytic processes. In such processes, molecules are excited by laser irradiation and dissipate their energy by tearing apart. Lasers in Industry, Von Nostrand Rheinhold Co., p. 312 (1972) suggests that laser-induced photolysis could be used to reduce the emission of gaseous pollutants from industrial stacks. An indicated drawback in such treatment of stack gases is that a high energy laser that emits UV radiation is required to effect photolysis of the common pollutants.
Ubhayaker and Williams, J Electrochem Soc, 123, 747 (1976) studied the sustained combustion of laser ignited carbon particles in quiescent N.sub.2 -O.sub.2 mixtures in test vessels. The reported tests indicate that particles greater than about 50.mu. could not sustain combustion after the ignition laser pulse.